Active transport of anionic and cationic drugs plays a critical role in their intestinal absorption, penetration across the blood-cerebrospinal fluid (CSF) and blood-brain barriers, and accumulation within excretory organs. In general, separate systems mediate the transport of charged organic solutes, the organic anion (OAT) and organic cation (OCT) transporter families. However, a number of drugs, e.g., the cation cimetidine, are transported by both systems. Using both Xenopus oocytes and cell lines expressing cloned drug transporters, we have examined their ability to handle several nucleoside phosphonate antiviral (NPA) drugs -- adefovir, cidofovir, and tenofovir. Of these drug transporters, only OAT1 and the ATP-driven drug pump, MRP2, transported the NPAs. None of the NPAs were transported by the OAT3 transporters from mouse, rat, or human. Thus, human nephrotoxicity of the NPAs reflects their avid (Km's ~5 uM) transport via OAT1. In the choroid plexus -- a component of the blood-CSF barrier, OAT3 is the predominant transporter expressed. Thus, choroid plexus shows limited accumulation of NPAs. This conclusion has now been confirmed in the OAT3 knockout mouse that shows no change in NPA transport despite nearly complete loss of the transport of mOAT3 substrates including taurocholate and estrone sulfate. This observation also indicates that other transporters, most likely the MRPs, are responsible for the limited penetration of the NPAs into the brain across the blood-brain and blood-CSF barriers. Our current focus is on the structural features of OAT1 and OAT3 that underlie their ability to descriminate between the NTAs and other OAT substrates shared by both isoforms. Molecular modeling based on the published crystal structures of related transport proteins have provided the basis for modeling of the binding pocket in OATs 1 and 3 and identification of amino acid residues putatively involved in substrate recognition and binding. These residues are currently being modified by site directed mutagenisis to assess their impact on transporter function.